This invention relates to the analysis of ignition voltage waveforms of internal combustion engines in general and more particularly to a circuit for generating a trigger blanking voltage useful in apparatus used to analyze such voltage waveforms.
The voltage waveform at the high voltage terminal of the ignition coil of an internal combustion engine provides important information for evaluating the ignition process. The ignition voltage waveform is characterised by the magnitude of the voltage present at certain points in time. Generally, analysis of the individual sections of the ignition voltage curve are initiated by a trigger signal taken therefrom. This generally corresponds to the high voltage present prior to breakdown in the spark plug. However, in some instances, during the course of ignition, the voltage curve exhibits a number of peaks which exceed the trigger threshold. As a result, after the initial triggering signal occurs, it is necessary to blank out further triggering signals which might occur during the course of ignition and which could reinitiate the measuring or analysis cycle. The required blanking interval is not necessarily a constant interval. In the ignition system of an internal combustion engine, the burning time of the ignition spark may be, for example, from 3.5 to 3.7 ms when idling. The burning time is normally followed by decay process lasting approximately 4 ms. The same engine at full load and high speed will have a spacing in time between two ignition processes of only 2.5 ms for example. In such a case, the duration of the burning line is shortened, due to insufficient charging of the ignition coil, to approximately 0.8 ms. At this high speed, the closing of the breaker contacts occurs during the delay cycle of the ignition coil. Thus, it becomes evident that trigger blanking must be made speed dependent.
In view of these requirements for a type of trigger blanking which is speed dependent, it is the object of the present invention to provide a circuit for generating a trigger blanking signal or pulse useful in the analysis of ignition voltage waveforms of an internal combustion engine, which trigger blanking voltage is of a duration which is speed dependent, becoming shorter with an increasing frequency of ignition pulses due to an increase in speed.